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Pseudomorphs

Inclusion and Pseudomorphs

The word inclusion desrcibes an enclosure, which means that something else is enclosed in something different. We see this expression in various fields. It is often used to describe inclusions in minerals and rocks. Important and generally valid is the fact that an inclusion is always as old or more often genetically older than the environment in which it is enclosed. In the sagenite long rutile needle crystals were formed before the quartz that surrounds them. Xenolites - older surface rocks in younger vulcanites, insects and various specks enclosed in amber ... Parent infusions and gases inside crystals are also older than the crystals themselves. In well-developed clear crystals, inclusions sometimes create fantastic patterns. They can degrade or on the contrary increase the value of precious stones, gems. The presence of real inclusions in agates is indeed possible, but so far insufficiently verified. Silicic acid solutions creating the agate matter are usually clear or colored with salts of metals, mainly iron and manganese. Salts and dirts in solutions can often create absurd shrubby and mossy phenocrysts in the growing agates. Occasionally, crystals other than quartz composition are first released in the almond-shape cavities. It is usually lamellar barite, limonite, hematite, goethite and calcite. Due to later inflowing quartz solutions, the mentioned minerals are subject to alterations and they usually turn into pseudomorphs – false form. Only the original mineral is subject to a chemical transformation, or it is completely removed, but the shape of the original crystal remains with all the external details. Framework "house" agates from Podkrkonoše localities are pseudomorphs of quartz after barite. Similarly, other processes of pseudomorphs will occur in agates under convinient conditions: limonite after hematite, goethite after limonite, calcite after aragonite etc. The final crystals are often coated with a thin layer (stocking) of other chemical compound, which usually colors them in many different ways. Beautiful shrubby and mossy structures similar to phenocrysts are caused also by the loss of molecular water of the original mineral. However, all potential chemical magics are nothing against the look to the inside of an open stone hiding a unique jewel - agate.

Author: Antonín Bílý
Illustrations: Akad. mal. Jitka Bílá